This application is directed to an integrated process that will remove metals and/or other contaminants from aqueous solutions rapidly, efficiently and continuously, and preferably convert metals to non-hazardous forms for recycling.
Water is used as a vehicle for the transport of soluble metals in many industrial processes. For example, large quantities of water are employed in mining, metal refining, plastics manufacture, metal finishing of all kinds, washing of contaminated soils or industrial harbor bottom spoils, etc. Not only are many of the metals mobilized in these aqueous solutions hazardous, but some are also valuable as a recyclable commodity or precious metal. The water itself may also be valuable, if it can be recycled back to the original process and/or employed for other industrial purposes. Thus, metals recovery can be quite desirable for economic reasons.
Recovery of metals and the water transporting them for re-use is also desirable for the maintenance of environmental integrity. As is commonly known, copper, chrome, silver and other metals in a soluble form are hazardous, above certain concentrations, to health and the environment. These same metals in metallic (solid) form, however, do not present as great a health or environmental problem; for example, drinking water can be channeled through copper pipes, silverware is used to serve food, and food is prepared in stainless steel cookware containing chrome and other alloy metals without adverse health effects. Unlike organic materials, which can be broken down by oxidation, microbial degradation or other methods, to harmless elemental components (such as carbon and nitrogen), metals are generally present in an aqueous medium in a dissolved (ionic) elemental form. Toxic metals remain potential environmental hazards in soluble form. The metals should thus be converted, as part of their recovery from water or other waste streams, to a metallic form, in which state their toxicity or potential hazard is reduced to near zero upon recovery.
The most common treatment for removal of toxic metals from solutions is to increase the pH with lime or sodium hydroxide. The metals then precipitate as "insoluble" salts in a "sludge". The metal sludges, although of a lower volume that the initial waste liquid, are still very hazardous. A common disposal method for sludges has been to bury them in hazardous waste landfills; however, buried sludges remain a threat to groundwater. This burial method is thus being quickly eliminated as a viable option, as governmental policy and legislation proscribe it.
Other methods for metals removal from aqueous waste streams (including reverse osmosis and ion exchange in packed beds) are severely hampered or rendered inoperable by suspended solids in the aqueous waste stream, even when these solids are only present in small amounts. Accordingly, the waste stream must be filtered prior to treatment. This can prove economically impractical for many waste streams; moreover, such treatment results in the deposition of some of the contaminant metal in the removed particles on the filter medium, which itself then becomes a hazardous waste.
Most advantageous of all methods for the recycling of waste metals would be one in which the metal ions are captured, concentrated, and reduced to their metallic forms through chemical or electrical means. Such a method would not only remove and separate the metals (and, optionally, other contaminants) from the waste stream, but also concentrate and render them non-hazardous by converting toxic ions to metallic forms.
Accordingly, it is an object of the present invention to provide a method for removal of waste materials from liquids contaminated with those materials, and an apparatus for use in conjunction with such method, that will overcome the disadvantages of prior art treatment methods with respect to handling of removed waste materials.
It is a further object of the invention to provide a method and apparatus for removal of waste materials from liquids which operate continuously in a steady-state condition, when such operation is desirable.
It is yet another object of the invention to provide a method and apparatus for removal of metals (and, optionally, other contaminants) from waste streams which, after an initial startup, permits metal capture, concentration and recovery (for example, by electroreduction) to be carried out simultaneously and continuously.